vec!() does not handle a trailing comma

``` rust
fn main() {
    println!("{}", vec!(1,2,3,).len());
}
```

gives the following error unlike the old syntax (`~[1,2,3,]`):

```
t.rs:2:30: 2:31 error: unexpected end of macro invocation
t.rs:2     println!("{}", vec!(1,2,3,).len());
                                    ^
```
